Androgens are known to mediate their effects through the androgen receptor (AR). Androgens play a role in a wide range of developmental and physiological responses, for example, male sexual differentiation, maintenance of spermatogenesis, and male gonadotropin regulation (R. K. Ross, G. A. Coetzee, C. L. Pearce, J. K. Reichardt, P. Bretsky, L. N. Kolonel, B. E. Henderson, E. Lander, D. Altshuler & G. Daley, Eur Urol 35, 355-361 (1999); A. A. Thomson, Reproduction 121, 187-195 (2001); N. Tanji, K. Aoki & M. Yokoyama, Arch Androl 47, 1-7 (2001)). Also, androgens are associated with the development of prostate carcinogenesis. Induction of prostatic carcinogenesis in rodent models has been associated with androgens (R. L. Noble, Cancer Res 37, 1929-1933 (1977); R. L. Noble, Oncology 34, 138-141 (1977)) and men receiving androgens in the form of anabolic steroids are reported to have a higher incidence of prostate cancer (J. T. Roberts & D. M. Essenhigh, Lancet 2, 742 (1986); J. A. Jackson, J. Waxman & A. M. Spiekerman, Arch Intern Med 149, 2365-2366 (1989); P. D. Guinan, W. Sadoughi, H. Alsheik, R. J. Ablin, D. Alrenga & I. M. Bush, Am J Surg 131, 599-600 (1976)). Furthermore, prostate cancer does not develop if humans or dogs are castrated before puberty (J. D. Wilson & C. Roehrborn, J Clin Endocrinol Metab 84, 4324-4331 (1999); G. Wilding, Cancer Surv 14, 113-130 (1992)). Castration of adult males causes involution of the prostate and apoptosis of prostatic epithelium (E. M. Bruckheimer & N. Kyprianou, Cell Tissue Res 301, 153-162 (2000); J. T. Isaacs, Prostate 5, 545-557 (1984)). This dependency on androgens provides the underlying rationale for treating prostate cancer with chemical or surgical castration (i.e. androgen ablation).
Prostate cancer is the second leading cause of male cancer-related death in Western countries (Damber, J. E. and Aus, G. Lancet (2008) 371:1710-1721). Numerous studies have shown that the androgen receptor (AR) is central not only to the development of prostate cancer, but also the progression of the disease to the castration resistance state (Taplin, M. E. et al. J. Clin. Oncol. (2003) 21:2673-8; and Tilley, W. D. et al. Cancer Res. (1994) 54:4096-4102). Thus, effective inhibition of human AR remains one of the most effective therapeutic approaches to the treatment of advanced, metastatic prostate cancer.
The AR possesses a modular organization characteristic of all nuclear receptors. It is comprised of an N-terminal domain, a central DNA binding domain, a short hinge region, and C-terminal domain that contains a hormone ligand binding pocket and the Activation Function-2 (AF2) site (Gao, W. Q. et al. Chem. Rev. (2005) 105:3352-3370). The latter represents a hydrophobic groove on the AR surface which is flanked with regions of positive and negative charges —“charge clamps” that are significant for binding AR activation factors (Zhou, X. E. et al. J. Biol. Chem. (2010) 285:9161-9171). Recent studies have identified a novel site on the AR called Binding Function 3 (BF3) that is involved into AR transcriptional activity.
It has been proposed a small molecule bound to the BF3 site could cause the AR protein to undergo an allosteric modification that prevents AR interactions with co-activators. Importantly, the BF3 site is located near, but distinct from, the ligand-binding site that is normally targeted by conventional anti-androgen drugs. Chemicals such as flufenamic acid (FLUF), thriiodothyronine (T3) and 3,3′,5-triiodo thyroacetic acid (TRIAC), can bind to the BF3 cleft, inhibit AF2 interactions and interfere with AR activity (Estebanez-Perpina, E. et al. Proc Natl Acad Sci USA (2007) 104:16074-16079). While these compounds revealed the importance of the BF3 site, they have shown a low potency (IC50>50 μM) and were found to bind non-specifically to the AR.
The activation of AR follows a well characterized pathway: in the cytoplasm, the receptor is associated with chaperone proteins that maintain agonist binding conformation of the AR (Georget, V. et al. Biochemistry (2002) 41:11824-11831). Upon binding of an androgen, the AR undergoes a series of conformational changes, disassociation from chaperones, dimerization and translocation into the nucleus (Fang, Y. F. et al. J. Biol. Chem. (1996) 271:28697-28702; and Wong, C. I. et al. J. Biol. Chem. (1993) 268:19004-19012) where it further interacts with co-activator proteins at the AF2 site (Zhou, X. E. et al. J. Biol. Chem. (2010) 285:9161-9171). This event triggers the recruitment of RNA polymerase II and other factors to form a functional transcriptional complex with the AR.
Notably, the current anti-androgens such as bicalutamide, flutamide, nilutamide and MDV3100, all target this particular process. However, instead of affecting the AR-cofactor interaction directly, these anti-androgens act indirectly, by binding to the AR ligand binding site. Thus, by preventing androgens from binding they also prevent conformational changes of the receptor that are necessary for co-activator interactions. While treatment with these AR inhibitors can initially suppress the prostate cancer growth, long term hormone therapy becomes progressively less effective (Taplin, M. E. et al. J. Clin. Oncol. (2003) 21:2673-8; and Tilley, W. D. et al. Cancer Res. (1994) 54:4096-4102). Factors that make the AR less sensitive to conventional anti-androgens include resistance mutations at the ligand binding site that can even lead AR antagonists to act as agonists further contributing to cancer progression (Chen, Y. et al. Lancet Oncol. (2009) 10:981-991).
Androgens also play a role in female cancers. One example is ovarian cancer where elevated levels of androgens are associated with an increased risk of developing ovarian cancer (K. J. Helzlsouer, et al., JAMA 274, 1926-1930 (1995); R. J. Edmondson, et al, Br J Cancer 86, 879-885 (2002)). The AR has been detected in a majority of ovarian cancers (H. A. Risch, J Natl Cancer Inst 90, 1774-1786 (1998); B. R. Rao & B. J. Slotman, Endocr Rev 12, 14-26 (1991); G. M. Clinton & W. Hua, Crit Rev Oncol Hematol 25, 1-9 (1997)), whereas estrogen receptor-alpha (ERa) and the progesterone receptor are detected in less than 50% of ovarian tumors.